There have been proposed many seat audio systems whose speaker is disposed in a car seat in order to increase sound effects in the car interior (for example, see Patent Literatures 1 and 2). A typical seat audio system includes a full-range speaker which is disposed near the headrest of a seat and can reproduce a low-to-high wide-range sound and a subwoofer which is disposed in a mid portion or a lower portion of the seat and can predominantly reproduce a low-frequency sound.
By disposing a subwoofer so as to be embedded in a seat, the seat vibrates in accordance with the level of a low-frequency signal of music, and the vibration is transmitted to the listener. A combination of a sound and vibration can provide higher realism to the listener. Typical examples of a subwoofer embedded in a seat include dynamic speakers, which use a cone paper or the like, and exciters, which output a sound by vibrating the contact surface.
Further, by outputting various types of warning sounds from a sound source, such a subwoofer can not only give an acoustic warning to the listener using a sound (warning sound) but also give a tactile warning to the listener using vibration. Thus, it is possible to increase the degree to which the listener recognizes the warning.